For instance, there has been proposed a system that facilitates communication among a plurality of apparatuses by connecting information apparatuses, such as personal computers, and electric apparatuses, such as television monitors, recorders, video players, and Internet Protocol (IP) telephones, with one another in buildings over a predetermined communication network. However, when data communication is performed through wired communication in buildings, cables or connectors used as transmission paths need to be typically installed at necessary positions. Therefore, a complicated construction work may be needed to construct communication systems.
Since commercial electric power sources, e.g., AC 120V (60 Hz) or 100V (50/60 Hz), are used in most houses, power lines used for supplying power are installed in the houses beforehand. Accordingly, when the power lines can be utilized as transmission paths for data communication, it is not necessary to newly provide communication lines and it is possible to secure the communication lines by connecting the communication apparatuses to outlets connected to the power source.
A power line communication (PLC) technology using the power lines for telecommunication has been disclosed in JP-A-2000-165304. In addition, various technology groups have been researching and developing the PLC technologies in predetermined frequency bands (for example, United States: 1.7 to 80 MHz, Japan: 2 to 30 MHz). In more detail, there has been proposed a method of using a plurality of subcarrier waves to generate multicarrier signals like an OFDM (orthogonal frequency division multiplexing) mode, and transmitting the multicarrier signals through the power lines.
However, due to the complicated wiring of power lines within buildings and different conditions of the buildings, the power lines have different transmission path capabilities according to locations. In addition, since various kinds of electric apparatuses are connected to the power lines, various kinds of noises may occur, and impedance is easy to vary. Thus, communication over the power lines is less likely to acquire a desired communication rate or more likely to have a poor communication quality due to a reduced S/N (signal-to-noise) ratio, as compared to communication over dedicated wired transmission paths.
Accordingly, in the power line communication, transmission path estimation is carried out on transmission paths between a transmission terminal and a receiving terminal at a predetermined timing before or during communication to measure transmission path characteristics, such as S/N, and transmission parameters are set so as to acquire a maximum transmission rate (bit rate) within an available range. At this time, a modulation factor (the degree of data overlap) on each carrier of a multicarrier signal is determined as the transmission parameter. When the transmission path has a favorable condition, the modulation factor is raised to increase the data transmission amount per unit time (to increase the bit rate). When the transmission path has a poor condition, the modulation factor is lowered to reduce the data transmission amount per unit time (to reduce the bit rate). Accordingly, it is possible to reduce an error rate to a predetermined value or less at the time of communication.
Meanwhile, when various electric apparatuses in houses are connected to one another through a network to transmit stream data, such as moving pictures or voices, it is necessary to secure QoS (quality of service) so that the data cannot be interrupted. In order to secure QoS in the power line communication, transmission band assurance needs to be performed, for example, by using a TDMA (time division multiple access) mode to allocate predetermined invariable timeslots on the basis of the maximum transmission rate for the transmission of stream data.
However, when the band assurance is performed by allocating the invariable time width of a timeslot on the basis of the maximum transmission rate of data in order to secure QoS with respect to specific data such as stream data, an unnecessarily large time width of timeslot may be secured at the timing when a great amount of data is not actually transmitted. Accordingly, since an available timeslot is limited in the transmission path, the timeslot may be insufficient, and thus the data may not be fully transmitted. In addition, the condition of the transmission path may vary in the power line communication with time. Accordingly, when an invariable time width of timeslot for the maximum transmission rate is secured to transmit data requiring QoS, other data may not be completely transmitted, if the transmission path is in a poor condition.
As described above, in a case in which data requiring QoS, such as stream data, is transmitted, when the band assurance is performed by allocating the invariable time width of timeslot on the basis of the maximum transmission rate of data in order to secure QoS with respect to specific data such as stream data, other data may not be fully transmitted due to an insufficient timeslot.